1. Field of the Invention
The present invention relates to reactive coating compositions for insulating and sealing surfaces, and more particularly, to coating compositions for sealing component layers of MLS gaskets used in internal combustion engines.
2. Discussion
A recurring challenge faced by designers of internal combustion engines is to maintain a gas-tight seal between the engine block and the cylinder head. In recent years, advances in gasket design—notably the introduction of multiple-layered steel (MLS) gaskets—have helped reduce sealing problems associated with the interface between the cylinder head and the engine block.
Conventional MLS gaskets typically comprise an interior layer that is sandwiched between a pair of exterior layers. The exterior layers are often made of 301 stainless steel, which is a comparatively strong metal having a high spring rate. The interior layer, which is also called a “spacer” layer, is normally made of less expensive materials, such as 409 stainless steel, or in some cases, zinc-plated steel or other low carbon steels.
Like other cylinder head gaskets, MLS gaskets include a number of apertures that extend between the exterior layers. When installed between the cylinder head and the engine block, the apertures circumscribe cylinder bores (i.e., combustion apertures), boltholes, coolant ports and oil ports. During engine operation, regions of the gasket adjacent to the cylinder bores are subject to greater stresses than portions of the gasket spaced further away from the combustion apertures. To compensate for the greater stresses, MLS gaskets generally include stopper layers, which surround each of the combustion apertures.
When compared to other regions of the MLS gasket, the stopper layers provide comparatively higher sealing pressure around the portions of the gasket that border the combustion apertures. In some cases the stopper layers comprise additional layers of metal, which are folded over or under the primary sealing layers (i.e., exterior layers or spacer layer). In other cases, the stopper layers comprise discrete annular rings positioned about the boundaries of the combustion apertures.
Most MLS gaskets also include secondary seals that, relative to the combustion apertures, are located radially outward of the stopper layer. Each of the secondary seals generally comprises an active spring seal that is defined by embossed beads on the external sealing layers. The embossed beads are normally arranged in pairs, so that a bead on one of the exterior layers has a corresponding bead on the opposing exterior layer.
MLS gaskets may also include a coating layer formed on sealing surfaces of one or more of the gasket layers. The coating layer helps improve the seal between the engine cylinder head and block. The coating layer is typically made of thermosetting polymers, such as nitrile butadiene rubber (NBR), fluorinated rubbers, and the like, which may be compounded with fillers, plasticizers, antioxidants and other materials that modify the properties and performance of the coating layer.
Though useful, conventional coatings used on MLS gaskets can be improved. For example, processes used to coat MLS gaskets are capital and energy intensive, requiring a large convection oven to cross-link (or cure) the coating precursors. Additionally, many existing coating formulations use volatile solvents, which present environmental challenges and add to the cost of the coating. Some coating systems also use primer and anti-stick coatings, which help the coating adhere to the surface of the metal gasket layers while permitting adjacent gasket layers to move relative to one another. However, the additional coating layers add to the cost and complexity of the coating process.
The present invention helps overcome, or at least mitigate one or more of the problems described above.